Reverted testsuite object files' home to 'obj'.

Details:
- Removed 'obj' and 'lib' from .gitignore.
- Added testsuite/obj/.gitkeep (which is an empty file).
- Updated testsuite/Makefile accordingly.
- Thanks to Vernon Austel for pointing out the .gitkeep trick to tracking
  empty directories in git.

.gitignore

@@ -8,8 +8,12 @@
 # -- compiler-related --
 
 # object files
+# NOTE: This will result in git also exluding the top-level obj directory
+# since its only contents are .o files.
 *.o
 # static library archives
+# NOTE: This will result in git also exluding the top-level lib directory
+# since its only contents are .a files.
 *.a
 # test executables
 *.x
@@ -19,11 +23,6 @@
 config.mk
 version
 
-# -- directories with build products --
-
-lib
-obj
-
 # -- makefile fragments --
 
 .fragment.mk

testsuite/Makefile

@@ -157,7 +157,7 @@ BLIS_LIB       := $(BLIS_LIB_PATH)/libblis.a
 #
 
 TEST_SRC_PATH  := src
-TEST_OBJ_PATH  := src
+TEST_OBJ_PATH  := obj
 
 # Gather all local object files.
 TEST_OBJS      := $(patsubst $(TEST_SRC_PATH)/%.c, \